A typical EGR system of an engine includes one or more EGR valves for controlling the flow of engine exhaust gas from the engine's exhaust system to the engine's intake system to meter a controlled quantity of exhaust gas into fresh air passing through the intake system where the air supports combustion of fuel in the engine's cylinders. The metered exhaust gas can limit in-cylinder temperature rise during combustion and consequently limit the quantity of oxides of nitrogen (NOx) in engine-out exhaust gas.
Some EGR systems, especially those designed for compression ignition (i.e. diesel) engines, have one or more heat exchangers for cooling recirculated exhaust gas. They are sometimes referred to as EGR coolers. Cooling of exhaust gas being recirculated can further limit the quantity of NOx in engine-out exhaust gas.